GIACIATION OF LOWER COPPER RIVER 465
nent one. Some measure of annual expense for new trestles and for raising the grade is to be looked for, even if the adjacent glaciers remain stationary or retreat slowly.
The second threat of extra expense of maintenance comes from the Copper River between the Flag Point and Hotcake Channels. There have been changes in the stream bed in even the short time since the bridges were built. While the steel spans were being erected the temporary trestles beside them constantly had to have new piles driven, because the current had undermined them. The steel bridges, built on concrete piers, seem safe, though not as safe as if it had been possible to erect the piers upon solid rock, rather than upon piling. The chief danger alluded to, however, is not that of undercutting by erosion, but of stream deposition. Unless artificially confined, the time will come when the several, heavily-loaded, Copper River distributaries will shift out of their present channels as they are silted up, leaving the expensive steel bridges high and dry, and necessitating new bridges wherever the new channels establish themselves. The railway engineers have to deal with what is to them a new type of stream and even the experience in attempting to control the lower Mississippi and in correcting small glacial streams in the Alps will fail to furnish adequate data as to what a source of trouble and expense a glacial stream may be to a railway and how it may best be coped with.
Fine Glacial Deposits of the Outer Edge of the Delta. We lack detailed information regarding the nature of the Copper River distributaries between the railway bridges and the sea. It is thought, however, that the grade continues to flatten, resulting in the deposition of the coarser silt carried in suspension by the river and causing the even shallower channels and tidal sloughs south of Flag Point, where the river is navigable only by small boats.
The bottom of Copper River is from 10 to 20 feet below sea level at the Flag Point and Round Island Channels, though slightly beyond the range of the tides and over twelve miles from the ocean.
Upon the edge of the Copper River delta the Coast Survey charts1 show a horizontal range of the tide of several miles. Offshore from the Scott and Sheridan Glaciers, for example, continuous tide flats are exposed for distances of from 5 to 7$ miles at mean lower low water. The vertical range of the tide is only ten feet, showing that the slope is very slight indeed. Crossing these tide flats are small channels or sloughs, navigable only by launches. More than two miles'from shore at the eastern edge of the delta, near the mouth of Martin River, there is a depth of only 1$ to 3 feet at mean lower low water. Opposite the middle of Copper River delta there are depths of only 54 to 102 feet nearly nine miles offshore, beyond which the water deepens to 414 feet in less than two miles. The bottom material is sticky mud, and the Pacific Ocean is discolored for over twenty miles offshore by the glacial silt in suspension, part of which goes westward and has helped fill Orca Inlet. The nature of the fine glacial sediment deposited on the outer part of the delta and exposed on the mud flats at low tide is well indicated by Seton Karr,q who tells of his experience in crossing the border of the delta in 1886 in a canoe with several natives. They were traveling inside a line of sandy islands, which furnished protection from the waves of the Pacific, and when the tide was too low to float the canoe any longer they were able to continue by sliding it over the slippery surface of the tide flats, continuing to sit in the canoe and push it
ĽU. S. Coast and Geod. Survey, Charts 8502, 8520, and 8513. "Seton Karr, H. W., Shores and Alps of Alaska, London, 1887, pp. 167-168, 171. 30